The 125I-diiodo derivative of alpha-bungarotoxin was used to determine the distribution of acetylcholine receptors on the surface of a clonal muscle cell line. Areas of elevated toxin-binding activity were identified and found to be free of cell debris or membrane invaginations. Iodinated toxin was also used as a probe of acetylcholine receptors in a non-fusing muscle line to determine the rates of turnover in the absence of innervation, to define three classes of receptor in the cell line, and to demonstrate differences in sedimentation coefficients existing between these classes of receptor. The basis for the observed difference in sedimentation coefficient is under investigation. Acetylcholine receptor has been purified from this cell line and is currently being used to stimulate antibody synthesis in rabbits. Antisera thus generated will be used to characterize the three classes of receptor in this cell line, to investigate the effect of antibody on receptor distribution, and to try to identify acetylcholine receptors in other tissues. BIBLIOGRAPHIC REFERENCE: Richman, David, P.. Patrick, James and Arnason, Barry G. W., Cellular Immunity in Myasthenia Gravis. The New England Journal of Medicine 294, 694 (1976).